The present invention relates to the manufacture of reinforced structures for parts of composite material, and more particularly for wheels, rims or rim elements.
Numerous methods exist for producing wheels or rims of composite material. Mention may be made, in particular, of high-precision injection molding of reinforced thermoplastic resins or of compression molding of thermosetting resins which are also reinforced.
These techniques require the use of staple fibers, which result in average mechanical properties of the finished products as well as their difficult mechanical optimization due to the displacements of the reinforcement fibers during the molding.
If it is desired to use continuous fibers, one can select an existing fabric. More industrially, one can also select the method of manufacturing composite parts known as R.T.M. (resin transfer molding) or any derivative process. In this process, the reinforcement materials, such as glass fibers, carbon fibers or fibers of any other type, are prearranged in the cavity of a mold. These reinforcement materials thus form the xe2x80x9cpreformxe2x80x9d of the final composite part. This operation of putting in place and structuring the reinforcements is known as xe2x80x9cpreformingxe2x80x9d. The resin is then transferred through the preform into the mold under vacuum and/or with a transfer pressure, and then cross-linked at suitable temperatures.
In the case of industrial manufacture, in order to optimize the use of the injection molds, the preforms can be prepared outside the injection mold. In this case the preforms are frequently made rigid, for instance by a thermoplastic binder, in order to facilitate their positioning in the injection molds.
The production of the preform is a critical step in the R.T.M. process, particularly for complex shapes having strong concavities, as is true in the case of tire nms.
French Patent 2595621 proposes manufacturing a preform of revolution for a part of composite material by placing a fibrous texture with deformable meshes, whether woven or plaited, on a preforming matrix having a shape similar to that of the structure to be produced, so as to fit the shape of the preforming matrix by auto-adaptation of the meshes. The process consists in placing the fibrous texture on the preforming matrix at an axial end of said preforming matrix, with the meshes having sides of opposite inclination with respect to the axial direction of the structure, and then gradually placing the fibrous texture on the preforming matrix in the direction towards the other end, while subjecting the texture to a tension in this axial direction.
This process leads to a symmetrical orientation of the reinforcement fibers of a fabric relative to the axial direction of the structure, which is not suitable in the case of tire rims, for which it is desirable to obtain a circumferential and axial orientation of the reinforcement fibers.
European Patent Application EP 0327173 presents a solution for the production of preforms of composite wheels by winding continuous fibers axially and then circumferentially. However, this process is slow and requires large and expensive equipment.
The object of the present invention is to present another, simple technique for the production of a rim preform from preassembled fibers. A structure of preassembled fibers can be a woven fabric, for example. Such a fabric is formed of warp threads and filling threads assembled by weaving.
In order to produce a preform with such a fabric, the use is to apply the warp and filling fibers in the desired directions of mechanical reinforcement, in this case the axial and circumferential direction. Under these conditions, the direct application of the fabric to a surface having long concavities, such as a rim, results in the appearance of wrinkles: the preforming cannot be realized.
The invention is a method for the production of a rim preform or rim-element preform having an axis of revolution by placing preassembled fibers along two directions defining deformable meshes on a preforming matrix in which:
a strip of said preassembled fibers is wound around a form of revolution in order to obtain a winding having a given number of layers of preassembled fibers;
said winding is fastened on a circumference of larger diameter of the preforming matrix so that the fibers are disposed, relative to the axial orientation, along a first orientation of angle alpha and a second orientation of angle beta which is smaller than alpha; and
applying said winding progressively until it covers the entire surface of the preforming matrix by subjecting it to tensions of substantially circumferential orientation and of a direction such that said tensions tend to increase said angle alpha.
The angle alpha of inclination of the first reinforcement fibers of the strip of the fabric is preferably between sixty-five and eighty-five degrees.
The method for the manufacture of a rim preform in accordance with the invention has the advantage of leading to orientations of the reinforcement fibers of the preform which are close to the axial and circumferential directions of the structure.
Preferably, after having applied the winding of the strip over the entire preforming matrix, this winding is compacted against the preforming matrix by the application of a radial pressure. This step has the advantage of increasing the content of reinforcement fibers in the finished composite part, which improves its mechanical properties.
Another object of the invention is a device for the manufacture of a rim preform or rim-element preform comprising:
a preforming matrix of outer profile of revolution corresponding to the inner profile of said preform;
means for fastening a winding of a strip of preassembled fibers along two orientations defining deformable meshes on a circumference of larger diameter of said preforming matrix;
means for applying said winding by subjecting it to tensions in a substantially circumferential direction.
The means for applying the winding advantageously comprise:
an application shoe having an application surface which is intended to come into contact with the winding over a given sector, said application surface having an entrance zone with an axial entrance profile the minimum diameter of which is at least greater than the larger outside diameter of said preform, an outlet zone with an axial outlet profile corresponding to the final outside profile of said preform, and a zone of transition having an axial profile developing progressively circumferentially from said entrance axial profile towards said axial outlet profile;
means for the holding and placing into relative rotation of the preforming matrix and said application shoe; and
means for applying said application shoe against said sector of said winding.
In a preferred embodiment, the circumferential development of a circumferential profile of the transition zone of the application surface of the shoe is all the faster the closer said circumferential profile is axially close to the circumference of larger diameter where the winding is to be initially attached.